1. Field of the Invention
The invention relates to a television signal processing apparatus, more particularly, to an outline enhancing circuit of a television signal which is applied to a television signal of one pseudo frame formed by interpolation from the television signal of one field.
2. Description of the Prior Art
In a video printer to obtain a hard copy of a television image, in the case where an image of one frame has been printed, a problem occurs in that a resultant image blurs due to the interlace scan. Thus, a pseudo frame image is produced from one field of the original television signal to be printed. As mentioned above, to form the pseudo frame signal from the field signal, it is necessary to produce the missing information by interpolation. As an interpolating method, it has been known to calculate a mean value between lines of the signal of the original field and to use the mean value as a signal of the other fields being interpolated.
FIG. 1 shows an example of a circuit to interpolate a mean value between lines. Reference numeral 31 denotes an input terminal. Input data Un is data of a source (original) field such as an even-numbered field, in which pixel data of columns in the vertical direction are located in accordance with the order from the upper position to the lower position. That is, data corresponding to the horizontal scan of the ordinary television scan is changed to data of the vertical scan. There are provided delay circuits 32 and 35 each having a delay amount corresponding to a sampling period of the pixel data on a time base in the vertical direction.
The input data Un and output data of the delay circuit 32 are added by an adder 33. An output of the adder 33 is supplied to a 1/2 magnifying circuit 34. An Output of the 1/2 magnifying circuit 34 is supplied to the delay circuit 35. The output of the 1/2 magnifying circuit 34 and an output of the delay circuit 35 are supplied to an adder 36. An output X.sub.n-1 of the adder 36 is taken out to an output terminal 37. The output X.sub.n-1 is used as data of the interpolated field, for example, the odd-numbered field.
A transfer function TFi(Z) of the interpolating circuit shown in FIG. 1 is expressed by the following equation. EQU TFi(Z)=1/2(Z+2+Z.sup.-1) (1)
Therefore, although the data of the even-numbered field as a source field has a flat frequency characteristic shown by reference numeral 38a in FIG. 2, the data of the odd-numbered field as an interpolation field is such that, as shown by reference numeral 38b, a gain decreases as a frequency rises and the gain is equal to 0 at a frequency of 1/4 fs (fs:sampling frequency on the time base in the vertical direction).
The above line mean interpolation can be realized by a very simple construction and is preferable from viewpoints of gradations, jerkiness, and the like as compared with a method of producing a frame signal by generating the signal of the same field twice. However, there are problems such that a vertical definition is low and the image blurs. Therefore, to improve an apparent sharpness, an outline in the vertical direction is enhanced.
FIG. 3 shows an example of a conventional outline enhancing circuit. Delay circuits 42 and 43 each having a delay amount of a vertical sampling period are serially connected to an input terminal 41. Input data X.sub.n and an output of the delay circuit 43 are supplied to an adder 44. An output of the adder 44 which has been transmitted through a 1/2 magnifying circuit 45 and an output of the delay circuit 42 are supplied to a subtracter 46. An output signal of the subtracter 46 is taken out to an output terminal 48 as an output signal W.sub.n-1 through an amplifier 47 of a gain b.
The conventional outline enhancing circuit executes a process of EQU W.sub.n =1/2b(-X.sub.n-1 +2X.sub.n -X.sub.n+1).
Due to this, the data of a total of three pixels comprising an object pixel as an object to be processed and upper and lower pixels adjacent the object pixel are digitally subjected to a quadratic differentiation. A transfer function TFe'(Z) from the input terminal 41 of the outline enhancing circuit to the output terminal 48 is as follows. EQU TFe'(Z)=1/2b(-Z+2-Z.sup.-1) (2)
The interpolating circuit shown in FIG. 1 and the outline enhancing circuit shown in FIG. 3 are serially connected for the data of the interpolation field (i.e., the interpolated field, in this case the odd-numbered field). Therefore, an overall transfer function from the input terminal 31 in FIG. 1 to the output terminal 48 in FIG. 3 is obtained by multiplying the transfer functions shown by the equations (1) and (2). EQU TFi(Z)TFe'(Z)=1/4b(-Z.sup.2 +2-Z.sup.-2) (3)
For the signal of the source field (even-numbered field) having the flat frequency characteristic 38a, the Output signal of the outline enhancing circuit has a frequency characteristic as shown at reference numeral 49a in FIG. 4 in which a high frequency band is emphasized. On the other hand, for the signal of the interpolation field (again, in this case the odd-numbered field) having the frequency characteristic 38b in which a high frequency band is reduced, the output signal of the outline enhancing circuit has a frequency characteristic shown at reference numeral 49b. For the latter case no quadratic differentiation component is generated and there is a problem such that the effect of the outline enhancement is not obtained. Thus, there is a problem such that the frequency characteristics are quite different between the two fields.